Exam practice

Fossils are the actual remains of organisms that lived millions of years ago, preserved in layers of sedimentary rock whose age can be measured. The oldest (lowest) layers contain only simple organisms, and the younger (upper) layers contain more and more complex ones, mammals last; fossils of extinct organisms and connecting links like Archaeopteryx show in-between forms. This is direct evidence that living things changed over time from simple to complex.

Food, space and shelter are limited, so not all the young can survive. They struggle with each other, with other species (predators, diseases) and with the environment. Only those with variations best suited to the environment survive and reproduce; most die before they can breed. So the population stays roughly the same.

Variations only supply differences between individuals. For a new species to arise, natural selection must keep the favorable variations and remove the unfavorable ones, the variations must be inherited (acquired, somatogenic variations are not), and this must go on over very many generations, often with mutations adding new characters and groups being separated. So variation is necessary, but on its own it does not make a new species.

Their forelimbs are homologous: a human arm, a horse’s foreleg and a bird’s wing are built of the same bones (humerus, radius, ulna, carpals, metacarpals, phalanges) arranged in the same way, though they do different jobs. Their early embryos are alike, with a tail and pharyngeal pouches, and they all share a backbone and the same body plan. Such deep similarities are best explained by descent from a common vertebrate ancestor.

Evolution is the slow, gradual change in living things over many generations, by which new species arise from earlier ones, generally from simple to more complex forms. For example, birds evolved from reptiles, and flowering plants from earlier seed plants.

A fossil is the remains, part or trace of a plant or animal that lived long ago, preserved in rock (a bone, a shell, a leaf print, a footprint). Fossils lie in layers of sedimentary rock, the oldest at the bottom. The lowest layers hold only simple organisms; higher layers hold more complex ones, and mammals appear only in the upper layers. Fossils of extinct and in-between forms, like Archaeopteryx (part reptile, part bird), show that living things changed over time.

A dead organism is carried by rivers into a lake or the sea and quickly buried in mud and sand. Its soft parts rot, but the hard parts (bones, teeth, shells, wood) remain. Layer upon layer of sediment piles up over millions of years and is pressed into sedimentary rock. Minerals in the water seeping through slowly replace the hard parts, copying their shape in stone (or the remains leave a mold that fills with minerals). Later, movements of the Earth may lift the rock up, and erosion exposes the fossil.

1. Fossils: older rock layers hold simpler organisms; e.g. ammonites, Archaeopteryx.
2. Comparative anatomy: homologous organs (human arm, horse’s foreleg, bat’s wing, whale’s flipper: same bones, different jobs) show common ancestry; analogous organs (wings of insects and birds) show similar solutions in different groups.
3. Vestigial organs: the human appendix, coccyx and ear muscles work in other animals and in our ancestors.
4. Connecting links: the platypus (reptiles–mammals), Archaeopteryx (reptiles–birds) and lungfish (fish–amphibians) have features of two groups.
5. Embryos: the early embryos of fish, salamanders, tortoises, chickens and humans look alike, with tails and pharyngeal pouches.

Darwin’s theory of natural selection (On the Origin of Species, 1859) has five points:

1. Enormous fertility: organisms produce far more young than can survive; numbers would grow geometrically, yet populations stay roughly constant.
2. Struggle for existence: the young compete for limited food, space and mates, and struggle against enemies and the environment.
3. Variation and heredity: individuals differ, and many variations are inherited.
4. Natural selection: those with variations best suited to the environment survive and reproduce (survival of the fittest); the others die out.
5. Origin of new species: favorable variations add up over many generations, until the descendants form a new species.

Natural selection is the process by which organisms whose inherited variations suit their environment best survive and reproduce more, and pass these variations on, while those with unsuitable variations die out. Nature, not people, “selects” them. Over many generations, it makes a species better suited to its environment and can lead to new species. Example: in a drought, giraffes with slightly longer necks reach the high leaves, survive and have young with long necks; those with short necks starve.

Organisms produce more young than can survive, so they struggle for existence. Their variations differ; those with favorable, inherited variations survive and reproduce (natural selection), passing the variations to their young. Generation after generation, the favorable variations add up. After a very long time, the descendants become so different from their ancestors, and from other groups of the same ancestor living elsewhere, that they form a new species.

1. It is a sudden change in the genes or chromosomes.
2. It brings a new character, not seen in the parents.
3. It is discontinuous: a clear jump, not a small step.
4. If it is in the egg or sperm, it is inherited.
5. It is caused by radiation, certain chemicals (mutagens) or errors in copying DNA.
6. Most mutations are recessive, and most are harmful or neutral; a few are useful.

Mutation creates new inherited characters that did not exist before, so it is the original source of new variations. Natural selection can only work on variations that exist; when a mutation happens to be useful in the environment, the organisms that carry it survive and reproduce more, and the mutation spreads through the population. Over many generations, many such mutations, kept by natural selection, add up to new species.

Mutations happen at random, in no fixed direction, and most are recessive and harmful or neutral. On their own they would not make species that fit their environment. Evolution also needs natural selection to keep the useful mutations and remove the harmful ones, the reshuffling of genes in sexual reproduction to make new combinations, and a very long time. So mutation supplies new raw material, but other factors, chiefly natural selection, shape it into new species.

Struggle for existence: every species produces far more young than the food, space and shelter can support, so the young must compete: with members of their own species (for the same food and mates), with other species (predators, parasites, competitors), and with the environment (cold, drought, floods). Most die.

Survival of the fittest: in this struggle, those whose variations suit their environment best (the “fittest”, meaning the best suited, not the strongest) survive and reproduce, passing their variations on; the rest die out. This is natural selection.

1. It doesn’t explain how variations arise or how they are inherited (Darwin knew nothing of genes).
2. It can’t explain why useful variations appear in some organisms and harmful ones in others.
3. It ignores mutation, a main source of new variations.
4. It can’t explain why useless or harmful characters, such as vestigial organs, still remain.

In layers of sedimentary rock, the oldest (lowest) layers, more than 3,500 million years old, contain only traces of single-celled organisms like bacteria and blue-green algae. Younger layers contain simple multicellular organisms, then invertebrates with shells, then fish, then amphibians and reptiles, and only the youngest layers contain birds, mammals and flowering plants. Fossils of in-between forms, like Archaeopteryx, link the groups. This order of appearance shows that today’s complex organisms developed, over a very long time, from primitive unicellular and multicellular ancestors.

• a.The fossil of Archaeopteryx, an animal that lived about 150 million years ago.
• b.It connects reptiles and birds (Reptilia and Aves).
• c.It had bird features (feathers, wings, a wishbone) and reptile features (teeth, claws on its wings, a long bony tail). Such a mix shows that birds evolved from reptiles: it is a connecting (missing) link between the two classes.

A. The hand has six fingers instead of five (polydactyly), a sudden new character: a mutation.
B. No. It appears only in a few individuals, those who carry the mutated gene.
C. A change in the genes or chromosomes (a mutation), caused by radiation, certain chemicals, or errors in copying DNA, and then inherited.
D. A mutation creates a new inherited character. If such a character helps an organism survive in its environment, natural selection keeps it, and it spreads through later generations; over a long time, many such changes can lead to new species.
E. Mutations (1) create new variations, the raw material of evolution; (2) can give useful features, such as resistance to a disease; (3) help species adapt when their environment changes; they are also used by plant breeders to make new crop varieties.